The performance of modern optical communication equipment deployed within a communication network must be monitored in a fashion which allows failures to be detected and located quickly and reliably so that immediate measures may be taken to bring the failed equipment back on-line. This minimizes the loss of service experienced by network users, and any loss of revenue to the network provider. Ideally, this monitoring should be capable of localizing any detected faults to a particular module within the network (a module being the smallest unit within the network that can be readily replaced in the event a fault associated with it is detected). In most applications a built-in automatic fault location scheme is essential.
In general, two basic performance monitoring techniques may be employed which allow for quick detection and localization of faults within multi-module optical communication systems and networks. A first technique provides a dedicated performance monitoring system within each module. This technique typically provides the fastest fault locating ability, but requires accessing electrical or optical signals at each system module so that various fault parameters can be measured. The cost of such performance monitoring systems is very high as separate monitoring equipment must be provided for each system module. Furthermore, this type of monitoring system is very lossy. The optical signal within the system must be accessed or "tapped" at each module so that is may be measured by the monitoring equipment, and every such tap introduces optical losses.
A second technique of performance monitoring utilizes a two-level monitoring system. In this system a maintenance signal of known characteristics is either added to the primary information signal being transmitted within the optical system on the same or slightly different wavelength as the primary information signal, or alternatively modulated on the primary information signal at a signal source prior to entering the system. This maintenance signal may, for example, be added at a previous transmitter or a previous optical amplifier. A preferred method of transmitting such a signal is disclosed in co-pending U.S. patent application Ser. No. 07/499,112 now U.S. Pat. No. 5,229,876, filed Mar. 28, 1990. The first level of performance monitoring is accomplished by a dedicated monitor at the input and output ports of the communication system. When this dedicated monitor detects a performance deterioration or a failure (indicated by an anormaly in the maintenance signal), an inter-module monitor is activated and sequentially connected to the input and output of each module within the failing communication system. This sequential switching and monitoring process goes on until the faulty module is identified. The fault will be located at the module which shows the proper maintenance signal at its input, but not at its output. Obviously, this two-level technique is more cost effective than a system in which each module has its own dedicated monitor. However, this two-level technique is relatively slow in locating a fault as a consequence of the switching which must be performed to connect the inner-module monitor the different modules within the system. In addition, the communication system must still be tapped at the input and output port of each module, and these taps add significant optical losses to the system.